The present invention relates to a new stereoselective method for the preparation of tris-O-substituted-(E)-1-(3,5-dihydroxyphenyl)-2-(4-hydroxyphenyl)ethene derivative of the general formula (I) which is a key intermediate in the synthesis of trans-resveratrol (I, R2=R2=R3=H). The invention also provides a method for the exclusive synthesis of trans-isomer of compounds of general formula (I) without any contamination of cis-isomer. 
Polyhydroxylated trans-stilbenes, the natural products, have been found to exhibit interesting biological activities. Trans-resveratrol has been found to raise the level of high-density lipoproteins (HDL) and lower the level of low-density lipoproteins (LDL) in human beings thereby reducing the risk of clogging arteries and consequent myocardial infarctions (Toppo, F. U.S. Pat. No. 6,048,903). Many 5-alkenyl resorcinols show antileukemic activity (Alonso, E; Ramon, D. J; Yus, M. J. Org. Chem. 1997, 62, 417-421; Thakkar, K; Geahlen, R. L.; Cushman, M. J. Med. Chem. 1993, 36(20), 2950-2955) and anti-tumor properties (He, K.; Zheng, Q. Y.; Zheng, B. L.; Kim, C. H. WO 2000/037021). Plant material containing trans-resveratrol has been used as a herbal medicine for the treatment of hyperlupemia and liver diseases in China and Japan for many centuries (Kimura, K. M. et al. Shoyqakugaku Zasshi 1987, 83, 35-58).
Further investigations led to identification of many important biological functions including inhibition of lypooxygenase activity (Kimma, Y. et al. Biochem. Biophys. Acta. 1985, 834, 275), inhibition of anaphylactoid (Ragazy et al. Pharmacol. Ref. Commun. 1988, 79, 20) and protection of lipoproteins against oxidative and free radical damage (Frankel E. N. et al. Lancet 1979, 1, 1017).
It has been reported that piceatannol, a known antileukemic principle in the seeds of Euphorbia lagascae, inhibits the PTK (protein-tyrosine kinase) activities of P40 as well as P56lck by binding to the substrate binding sites (Geahlen, R. L.; McLaughlin, J. L. Biochem. Biophys. Res. Commun. 1989, 165, 241-245). Thakkar et al. studied a series of hydroxylated trans-stilbenes and reported enhanced activity for inhibition of the lymphoid cell lineage-specific protein-tyrosine kinase P56lck which plays an important role in lymphocyte proliferation and immune function.
In view of the vital biological activities of the polyhydroxylated trans-stilbenes, there have been a few methods reported for the synthesis of these compounds. For example, trans-resveratrol is synthesized by the Wittig reaction of 3,5-dimethoxybenzyltriphenyl phosphonium salt with p-anisaldehyde in the presence of n-butyl lithium. The mixture of cis-and trans olefins so obtained is demethylated with large amount of boron tribromide to get very low yield of the product (Toppo, F. U.S. Pat. No. 6,048,903). This method suffers from the low yield, low quality and use of expensive and hazardous reagents, which prevents from employing this method on commercial scale. Similar low yields are reported employing condensation of phosphonate esters with aromatic aldehydes followed by demethylation using pyridine hydrochloride (WO 2000/021368). In case of piceatannol, tetramethoxystilbene precursor is synthesized by the Witting reaction and the methyl groups are then removed by demethylation reaction with pyridine hydrochloride (Drewes, S. E; Fletcher, I. P. J. Chem. Soc. Perkin Trans. I, 1974, 961-962) or with boron tribromide (Bajaj, R; Gill, M. T.; McLaughlin, J. L. Rev. Latinoamer Quim. 1987, 18, 79-80).
Use of trimethylsilyl protecting groups followed by witting reaction to get the hydroxy-trans-stilbene products has also been reported (Reimann, E. Tetrahedron Lett. 1970, 4051-4053). Yet another route comprising the reaction of 3,5-dihydroxyphenylacetate and 3,4-dihydroxybenzaldehyde in acetic anhydride, decarboxylation of the ensuing 3,3xe2x80x2, 4,5xe2x80x2-tetraacetoxystilbene-xcex1-carboxylic acid with copper and quinoline at high temperature, and hydrolysis of the resulting piceatannol tetraacetate with sodium hydroxide is also reported. (Cunningham, J; Haslam, E; Haworth, R. D. J. Chem. Soc. 1963, 2875-2883). All these processes for the preparation of trans-resveratrol are reported to proceed in moderate to low yields.
In those cases where Wittig reactions are employed, the undesired cis-isomers are also formed along with the desired trans-isomers to the extent of 52% (trans 48%) when potassium t-butoxide is employed (Ali, M. A.; Kondo, K; Tsuda, Y. Chem. Pharm. Bull. 1992, 40(5), 1130-1136). Similarly, cis isomers are formed to the extent of 45% (trans:23%) when sodium hydride is used. (Cushman, M; Nagarathnam, D; Gopal D. et al. J. Med. Chem. 1992, 35(12), 2293-2306; Chen, Yi-Ping; Lei, Tong-Kang. Zhongquo Yiyao Gongye Zazhi 2000, 31(7), 334-336.
In another process, 3,5-dimethoxybenzyl trimethylsilyl ether is treated with lithium metal in the presence of p-anisaldehyde to yield, after dehydration, 3,4xe2x80x2,5-trimethoxystilbene, a penultimate intermediate which requires excess boron tribromide (Alonso, E; Ramon, D. J.; Yus, M. J. Org. Chem. 1997, 62(2), 417-421). Similar use of excess boron tribromide has been indicated in WO 01/60774.
A wide variety of demethylating reagents such as boron tribromide, boron tribromide-dimethyl sulfide complex, boron trichloride-dimethyl sulfide complex, pyridine hydrochloride with a catalytic amount of quinoline at elevated temperatures, methyl magnesium iodide etc., are employed. In all these cases, demethylation is always incomplete and the yields and quality are disappointing. To purify the product obtained after dealkylation, costly chromatographic techniques were employed (Thakkar, K; Geahlen, R. L; Cushman, M. J. Med. Chem. 1993, 36(20), 2950-2955).
In the synthesis, which involves the Wittig reaction, the mixture of cis- and trans-isomers are separated by column chromatography or irradiated a solution of the mixture to convert cis into trans isomers.
These processes are commercially and technically difficult to be implemented. In addition, main problems in the synthesis of these polyhydroxylated-1,2-diphenylethenes are caused by the instability of this polyphenolic stilbene due to its oxidation, resulting in the formation of unstable radicals and quinones. The final isolation of the product has to therefore be carried out from dark coloured mixture containing multiple components.
The objective of the present invention is to develop a commercial method for preparing polyhydroxy-1,2-diphenylethenes, especially (E)-1-(3,5-dihydroxyphenyl)-2-(4-hydroxyphenyl)ethene, with exclusive trans stereo chemistry.
Another objective of the present invention is to overcome the technical difficulties associated with the reported methods and develop a commercial process to manufacture exclusively the trans-isomer.
Yet another objective of the present invention is to provide a simple and effective process without using photochemical reaction in the manufacturing process.
Still another objective is to avoid the use of expensive and hazardous chemicals viz. boron tribromide, lithium, Lithium aluminum hydride, sodium hydride, triethyl phosphite, phosphorus tribromide, etc. in the present process.
The present invention relates to a process for the stereoselective synthesis of tris-O-substituted-(E)-1-(3,5-dihydroxyphenyl)-2-(4-hydroxy phenyl)ethene derivatives of general formula (I), wherein those disadvantages, which have plagued this art, are obviated. 
wherein R1, R2, and R3 are independantly represented as alkoxyalkyl, allyl, vinyl, silyl, formyl, acyl, arylalkyl or substiuted arylalkyl groups or a combination thereof.
Moreover, the foregoing technique has been found to be markedly attractive, both from commercial point of view, as well as from a stereo selectivity standpoint, and affords an almost exclusive formation of the trans-isomer and is also free from the limitations discussed above.
The subject compounds of general formula (I) are eminently well suited as intermediates to manufacture (E)-1-(3,5-dihydroxyphenyl)-2-(4-hydroxyphenyl)ethene (I; R1,R2,R3=H) for carrying out the process of the invention. Moreover, the preparation of this can be accomplished quite easily and inexpensively.
Accordingly, the compounds of general formula (I) of the present invention are useful as intermediates for the production of (E)-1-(3,5-dihydroxyphenyl)-2-(4-hydroxyphenyl)ethene (I; R1,R2,R3=H) which has broad spectrum biological activity.
The present invention relates to a stereoselective process for the preparation of substituted diaryl ethenes and more specifically tris-O-substituted-(E)-1-(3,5-dihydroxyphenyl)-2-(4-hydroxy phenyl)ethene derivative of general formula (I), 
wherein R1, R2 and R3 are independently represented as alkoxyalkyl, allyl, vinyl, silyl, formyl, acyl, arylalkyl or substituted arylalkyl groups or a combination thereof, the said process comprising steps of:
a) treating compound of formula (III) with halogenating agent in an organic solvent under nitrogen atmosphere for a period of 60-180 minutes, 
wherein R1, R2 and R3 are as defined above,
b) removing the precipitated byproduct of step (a) by filtration to get a solution.
c) evaporating the solution of step (b) to remove the solvent, to obtain a residue,
d) dissolving the residue of step (c) residue in an dry organic solvent,
e) treating the solution of step (d) with an oxidizing agent, at a temperature between xe2x88x9230xc2x0 C. to +60xc2x0 C. for a time period of 15-300 minutes,
f) quenching the reaction mixture of step (e) with ice, thereby forming an organic layer and an aqueous layer,
g) separating the organic layer of step (f),
h) washing the organic layer of step (g) with an aqueous alkali bicarbonate solution followed by water,
i) charcoalizing the organic layer of step (h), filtering and removing the organic solvent under vacuum to obtain compound of general formula (IV) 
wherein X is halogen and R1 to R3 are defined as above
j) treating the compound of formula (IV) of step (i) with a phase transfer catalyst and a base in an organic solvent at a temperature in the range of xe2x88x9260xc2x0 to +110xc2x0 C., for a period of 15-480 minutes,
k) adding water to the reaction mixture of step (j),
l) separating the organic layer from the reaction mixture of step (k), charcoalizing, filtering, and evaporating the organic layer to obtain a residue, and
m) treating the residue of step (l) with alcoholic solvent at a temperature range of 25xc2x0-32xc2x0 C. for 1-3 hours to obtain exclusively a stereoselective trans form tris-O-substituted (E)-1-(3,5-dihydroxyphenyl)-2-(4-hydroxyphenyl)ethene derivative represented by general formula (I).
In another embodiment, the compound of general formula (I), is alternatively prepared by a process comprising steps of oxidising compound of formula (III) using an oxidising agent in a medium containing aqueous organic acid at a temperature in the range of xe2x88x9230xc2x0 to +60xc2x0 C. for a period of 15-300 minutes, followed by quenching the reaction mixture into ice and performing steps (g) to (l) of claim 1 to obtain compound of formula (II), which is further treated with a phase transfer catalyst, halogenating reagent and in an organic solvent at a temperature in the range of xe2x88x9260xc2x0 to +110xc2x0 C. for a period of 15 to 480 minutes, followed by adopting steps (k) to (m) of claim 1 to obtain exclusively stereoselective form of tris-O-substituted (E)-1-(3,5-dihydroxy phenyl)-2-(4-hydroxyphenyl)ethene derivatives represented by general formula (I). 
The process adopted for the preparation of compound of formula (I) is depicted in a flow chart (Scheme I) as shown below: 
The groups, R1, R2 and R3, which represent a protecting group for the hydroxyl groups in the compounds of the formulae (I)-(IV) are arylalkyl, allyl or silyl groups which can be removed selectively without disturbing the double bond and more particularly, the stereochemistry with respect to the double bond. Since the protecting group ultimately is to be cleaved, it is desirable that the protecting group be easily cleaved in an optional step after the preparation of compound (I). It is further required that the protecting group be stable during the olefination reaction during halogenation and alkali-treatment. In this respect arylalkyl, allyl, vinyl and silyl groups are useful protecting groups. In addition alkoxymethyl, acetyl and formyl groups are the alternate choices.
In another embodiment, the present invention provides the use of 3,4,5-tri-O-substituted dibenzyl sulphide as a starting material for the synthesis of tris-O-substituted-(E)-1-(3,5-dihydroxyphenyl)-2-(4xe2x80x2-hydroxyphenyl)ethene derivative and trans-reseveratrol.
In one another embodiment, the present invention provides method for performing the oxidation reaction at a temperature in the range of xe2x88x9230xc2x0 C. to +60xc2x0 C. and preferably 15xc2x0 to 25xc2x0 C.
In an another embodiment, the present invention provides the use of an oxidizing agent for carrying out oxidation selected from the group consisting of hydrogen peroxide, potassium permanganate, peracetic acid, trifluoroperacetic acid and m-chloroperbenzoic acid.
In one another embodiment, the present invention provides the use of an halgenating reagent for carrying out halogenation reaction selected from the group consisting of chlorine, carbon tetrachloride, sulfuryl chloride, phosphorous (V) chloride, N-chlorosuccinimide and N-bromosuccinimide.
In a further embodiment, the present invention provides temperature for performing the halogenation reaction in a range of xe2x88x9260 to +110xc2x0 C. and preferably between 20xc2x0-45xc2x0 C.
In yet another embodiment, the present invention provides the use of organic solvent in carrying out the reaction selected from the group consisting of chloroform, dichloromethane, benzene, toluene, tetrahydrofuran and dioxane.
In still another embodiment of the present invention, base used is selected from the group consisting of alkali metal hydroxide, alkaline earth metal hydroxide, and secondary/tertiary amine organic bases.
In still yet another embodiment, the present invention provides the use of phase transfer catalyst to promote the reaction, preferably lipophilic phase transfer catalyst selected from the group consisting of quaternary ammonium salts and crown ethers.
In still another embodiment, the present invention provides the use of preferably equimolar ratio of halogenating agent and the substrate (Process B).
In still another embodiment, the present invention provides the use of excess halogenating agent to enhance the rate of reaction and quality of the product (Process A).
In still one more embodiment, the present invention provides the relative mole equivalents of compound (II) and base as in the range of 1:1 to 1:4.
In still another embodiment, the present invention provides the reaction time for completion in the range of 15 minutes to 480 minutes.
In one more embodiment, the present invention provides the preparation of an intermediate xcex1-halosulphone derivative of formula (IV) in the Process B.